1.3 Word learning
1.3.4 Rapid neural dynamics for exposure to novel words
While studies using semantic contexts to learn new lexical items showed that the acquisition of novel lexico-semantic forms can take place within only a few exposures (Mestres-Missé et al., 2007; Borovsky et al., 2010), it remained unknown whether semantic associations are necessary for the successful learning of novel word-forms or if rapid learning of mere phonological forms could occur. First evidence indicating rapid perceptual learning of novel word-forms without imposed or acquired meanings were shown with an indirect measure of learning, an ERP response enhancement to novel words through 160 repetitions (Shtyrov et al., 2010b). At the same time, the neural response to known words showed no significant change, with an inclination to decrease over the course of exposure. The rapid learning effect was underpinned by enhanced source activation in the left-lateralised perisylvian cortex. This cortical correlate of learning, counteracting the suppression usually occurring in response to repeated auditory stimulation (Haenschel et al., 2005; Garrido et al., 2009), was suggested to demonstrate rapid formation of a new neuronal circuit for the newly
introduced linguistic item. The effect was replicated with more tokens and showed specificity to linguistic material as exposure to an unfamiliar non-speech stimulus, modified from the speech signal in its acoustic properties, did not establish response changes (Shtyrov, 2011). In these two studies, subjects ignored the novel spoken word-forms and focussed their attention on visual stimulation, and, therefore, it can be suggested that the neural learning took place without conscious effort. The response exhibiting the increase occurred at ~70-140 ms after word divergence point (Shtyrov et al., 2010b; Shtyrov, 2011), matching the latencies of lexical access reported in earlier studies (see Section 1.2.1). Similar learning-related dynamics was acquired in a tonal language (Yue et al., 2014).
The results show marked resemblance with the automatic spoken word recognition results (reviewed in Section 1.2.1.), indicating early automatic memory-trace formation for novel lexical items. The studies did not, however, include any measures of behavioural memory performance in order to investigate the correspondence between the rapid neural dynamics and subsequent recollection of the novel words.
The automaticity account also bids further examination on how focussed attention on the stimuli may affect the neural processing changes during exposure. Furthermore, several other cognitive factors may influence the individual neural patterns of response development, possibly revealing more detailed information on the plastic capacity and function of the neural circuits within the language network of the brain.
2 AIMS OF THE THESIS
The general aim of the current thesis was to investigate the effect of short- and long-term exposure to spoken words on brain responses that presumably reflect word memory-trace activation in the human neocortex. Cortical processing of known as well as novel words (pseudo-words) with different kinds of phonological as well as exposure-related properties was investigated in adults and children using high-resolution EEG in combination with behavioural and neuropsychological measures.
Using acoustically and phonetically carefully matched spoken stimuli, the neural responses reflecting word recognition were obtained by time-locking the ERPs to the divergence points that distinguished the tokens from each other as well as from other items in the lexicon. Neural dynamics of the different stimulus types were studied in conditions with distinct demands of focussed attention on the stimuli, and in subjects with varying language experience and reading ability. The aim was to determine the connections between various putatively influential background factors and the neural dynamics elicited by the experimental word-forms.
More specifically, the aim of STUDY I was to determine the effect of word frequency, i.e. the occurrence of a word in a language, on the strength and temporal dynamics of memory-trace activation for spoken words. This was attained by contrasting ERPs to spoken words with higher vs. lower frequency, and their pseudo-word analogues. In the Hebbian learning framework, the more a pseudo-word is used and encountered, the stronger the internal connections of the memory trace develop. This assumption leads to a direct hypothesis of stronger neural memory-trace activation for words with high frequency of occurrence and weaker activation for less frequent words. Hence, greater negative-going response amplitude, presumably reflecting stronger memory traces, was expected for words with higher frequency as opposed to those with lower frequency. Moreover, pseudo-words were expected to elicit weaker response than the words. Furthermore, contrasting the responses to the different lexical types, the speed of lexicality processing was assessed. This way, lexical access in the brain for known words differing in the extent of long-term exposure, and contrastively, brain activations for pseudo-words that lack such existing memory traces were examined.
STUDYII investigated rapid formation of memory traces for novel words through brief (~30 min) but extensive perceptual exposure. Responses to both novel and known word-forms at early and late stages of exposure were compared, and an increase in response negativity was assumed to reflect memory-trace formation, as indicated by previous research (Shtyrov et al., 2010b; Shtyrov, 2011; Yue et al., 2014).
To test this assumption, memory performance on the word stimuli after exposure was measured with free recall and recognition tasks. Possible differences in how phonological make-up of spoken stimuli affects the neural dynamics was scrutinised by presenting novel word-forms with either native or non-native phonology. Notably, no semantic meaning was assigned or learnt for the novel words, with the aim to study purely phonological-lexical word-form processing. Moreover, the effect of attention was investigated by modifying its direction in two listening conditions, in which the subjects either ignored or attended on the spoken stimuli. Attention was hypothesised not to have a considerable effect on the rapid neural increase for novel words due to previously shown robust increase for ignored novel words. Further aimed at validating the proposal that the neural response increase really indicates word memory-trace formation, individual neural response changes to novel words were regressed on the measures of memory performance.
STUDYIII probed possible effects that previous language experience may have on the exposure-induced neural dynamics for novel words. Factors delineating experience in learning non-native languages, i.e. the number of languages and their learning onsets and acquired proficiencies, were obtained and regressed against individual neural increase to novel non-native and native word-forms. Based on previous reports of second language learning shaping the language networks in the brain, it was hypothesised that experience in multiple languages besides mother tongue would show beneficial effects in learning novel words with unfamiliar phonology. Modelling the influence of the background factors separately for novel words with either familiar or unfamiliar phonology, the aim was to determine if previous foreign-language learning experience is associated with novel word-learning capacity in general or in a specific manner depending on the phonological familiarity of the word input.
In STUDY IV, neural responses to an auditorily repeated novel word-form with native phonology were investigated in children with or without dyslexia. Children
with typical reading and writing skills (controls) were expected to show rapid increase in the response to the novel word, similarly to adults, or even faster. On the basis of the phonological processing and word learning deficits reported in dyslexia, the short passive exposure (11 min) to the novel word-form was hypothesised to show an impaired lexical memory-trace formation dynamics compared to the controls.